Cretaceous alkaline intra-plate magmatism in the Ecuadorian Oriente Basin: Geochemical,...

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Earth and Planetary Science L

Cretaceous alkaline intra-plate magmatism in the Ecuadorian

Oriente Basin: Geochemical, geochronological and

tectonic evidence

Roberto Barragana,*, Patrice Babya, Robert Duncanb

aInstitut de Recherche pour le Developpement (ex-ORSTOM), Unite de Recherche 104, Universite Paul Sabatier, Toulouse III,

38 rue des Trente-Six Ponts, 31400 Toulouse, FrancebCollege of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Or 97331, USA

Received 27 September 2004; received in revised form 15 March 2005; accepted 31 March 2005

Editor: E. Bard

Abstract

Small volumes of Cretaceous alkaline basaltic magmas have been identified in the sedimentary infill of the Ecuadorian Oriente

foreland basin. They are characterized by a restricted range of compositional variation, low LILE/HFSE ratios and Sr–Nd isotope

values within the range of oceanic island basalts (OIB). Reflection seismic data show that a pre-existing NNE–SSW Triassic and

Jurassic rift controls the location and occurrence of these alkaline eruptive sites. Radiometric ages (40Ar–39Ar, incremental heating

method) and the biostratigraphic record of their surrounding sediments indicate a NNE–SSW systematic age variation for the

emplacement of this alkaline volcanism: from Albian (110F5.2 Ma) in the northern part of the Oriente Basin, to Campanian

(82.2F2.0 Ma) in the west-central part. The geochemical, geochronological and tectonic evidences suggest that asthenospheric

mantle has upwelled and migrated to the SSW, into the region underlying the pre-existing Triassic and Jurassic rift (thin-spot?).

We propose that subduction was abandoned, subsequent to the accretion of allochthonous terranes onto the Ecuadorian and

Colombian margin in the latest Jurassic–earliest Cretaceous, causing the relict slab material, corresponding to the eastwards-

directed leading plate, to roll-back. Unmodified asthenospheric mantle migrated into the region previously occupied by the slab.

This resulted in partial melting and the release of magmatic material to the surface in the northern part of the Oriente Basin since

at least Aptian times. Then, magmatism migrated along the SSW-trending Central Wrench Corridor of the Oriente Basin during

the Upper Cretaceous, probably as a consequence of the lateral propagation of the transpressive inversion of the Triassic–

Jurassic rift. Eventually, the Late Cretaceous east-dipping Andean subduction system was renewed farther west, and the

development of the compressional retro-foreland Oriente Basin system halted the Cretaceous alkaline magmatic activity.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Northern Andes; Ecuador; Oriente Basin; alkaline magmatism; Cretaceous; roll-back; transpressive inversion

* Corresponding author. OXY Ecuador, Ave. Naciones Unidas E7-95 y Shyris, Edificio Banco del Pacıfico, Quito-Ecuador. Tel.: +593 2 299

0012-821X/$ - s

doi:10.1016/j.ep

3700; fax: +593

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etters 236 (2005) 670–690

ee front matter D 2005 Elsevier B.V. All rights reserved.

sl.2005.03.016

2 299 3701.

ss: [email protected] (R. Barragan).

R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690 671

1. Introduction

Intra-continental plate alkaline magmatism is

reported from numerous locations in Mesozoic and

Cenozoic strata along the Pacific margin of the

Americas and the Antarctic Peninsula [1–11]. Dif-

ferent models, each related to a tectonic setting, have

been proposed to explain the generation and occur-

rence of alkaline magma. Possible mechanisms

include: (a) mantle plumes with abnormally high

asthenospheric temperatures beneath continental

crust [12–14] (e.g., Columbia River Basalts in the

NW United States [15]); (b) upwelling and decom-

pressional melting due to lithospheric extension [16]

and rift propagation (e.g., Andean Jurassic back-arc

ECUADOR

Guayaquil

Quito

Talara

ORIENTEBASIN

MARAÑONBASIN

PUTUMAYO BASIN

Real Cordillera

Subandean Syste

Central Corridor

CUTUCU UPLIFT

NAPO UPLIFT

PASTAZADEPRESSION

JAG

Y

DA

SUMACO

REVENTADOR

ANTISANA

CAYAMBE

SUB-ANDEAN FORELAND BASIN

GU

AY

AN

AS

HIE

LD

PUNGARAYACU

Fig. 1. Regional location map. Synthetic tectonic map of

basin between 258 S and 08 [1–3]); (c) development of

slab windows associated with the ending of subduc-

tion processes along active continental margins fol-

lowing ridge crest–trench collision [17–19] (e.g., Baja

California [4]; Southern Patagonia [5–7]; Antarctic

Peninsula [8 9]; British Columbia [10]); and (d) a

combination of slab roll-back associated with lateral

and vertical asthenospheric migration into the locus

of a pre-existing area of lithospheric thinning or

bthin-spotsQ (e.g., The Antarctic James Ross Islands

[11]).

Here we present evidence of active intra-plate alka-

line magmatism from the Oriente Basin of Ecuador

associated with the evolution of the northwestern

margin of the South American plate during Cretaceous

m

Eastern Inverted System

Cretaceous Igneous Bodies

Active volcanoes

0 100 Km

MARAÑON BASIN (PERU)

ORIENTE BASIN (ECUADOR)

PUTUMAYO BASIN (COLOMBIA)

TAPI

VISTA

LAGUNA

AUCAUAR

URALPA

YUNO

77 00' W

0 00'o

o

the Ecuadorian Oriente Basin (modified from [20]).

CHAMBIRA/CURARAY

Foreland Isostatic Rebound

TRANSPRESSIVE INVERSION

CRATONIC BASIN

TECTONIC EVENT

Initiation of Foreland

ARAJUNO continental

ORTEGUAZA

UP. TIYUYACU

continental

shallow marine

CHALCANA

MERA-MESA continental

LOW. TENA

LOW.TIYUYACU

UP. TENA continental

continental

UP. NAPO

LOW. NAPO

shallow marine

shallow marine

HOLLIN

Basal Tena

shallow marinecontinental

continentalTRANSPRESSION

PALEOCENE

0

10

20

30

40

50

60

70

80

90

100

110

NE

OG

EN

EP

ALE

OG

EN

EC

RE

TA

CE

OU

S

PLIOCENE

MIOCENE

OLIGOCENE

EOCENE

LATE

EARLY

CHRONO- STRATIGRAPHY

ROCK UNIT PALEO-

ENVIRONMENTLITHOLOGY

82.2 2.0 (40Ar/39Ar)

PR

E-O

RO

GE

NIC

M

EG

AS

EQ

UE

NC

EF

OR

ELA

ND

ME

GA

SE

QU

EN

CE

106 5 (40Ar/40K)

v

v

v

v

v

v

v

v

v

continental/shall marine

continental

91.2 4.6 (40Ar/39K)

v

v

R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690672

times. The Oriente Basin is part of the Amazonian

retro-foreland basin system of the present-day north-

ern Andes (Fig. 1). The magmatism reported here

developed during deposition of a Mesozoic sedimen-

tary series that was deposited within relatively stable

shallow marine conditions, corresponding to the

Aptian to Campanian, Hollın and Napo formations

[21,22]. Furthermore, the geographic distribution of

this alkalic igneous activity is largely confined to a

major transpressive NNE–SSW corridor, in the central

part of the Oriente Basin, which formed by tectonic

inversion of an upper Triassic–lower Jurassic rift,

starting in the Late Cretaceous [20].

This paper presents geochemical, geochronological

and tectonic data aimed at understanding the origin

and evolution of this intra-plate magmatic activity. The

focus is not only on the origin of this small volume,

mafic igneous province, here termed bOriente Basin

Basaltic volcanismQ (OBB), but also on its relation-

ship with the geodynamic evolution of north-western

South America.

RIFTING

SUBDUCTION ONSET

BACK-ARC EXTENSION

120

130

MIDDLE-UPPER JURASSIC

MISAHUALLI / CHAPIZA

continental

LOWER JURASSIC

TO UPPER

TRIASSIC

coastal plain to shallow

marine platform

SANTIAGO

BAS

EMEN

T

228

volcanoclastic

PR

E-C

RE

TA

CE

OU

S

ME

GA

SE

QU

EN

CE

PALEOZO

IC

v v

vv

vv

vv

vv

vv

vv

v

175

vv

Fig. 2. Stratigraphic column for the Ecuadorian Oriente Basin

indicating major tectonic and magmatic events (modified from

[26,27]).

2. Geological setting

The Ecuadorian Oriente Basin [23] forms part of

the large present-day Maranon–Oriente–Putumayo

foreland system [24], developed between the Pre-

Cambrian Brazilian–Guyana basement shields to the

East, and the Andean Cordillera to the West (Fig. 1).

The Oriente Basin preserves a sedimentary infill, ran-

ging in age from Paleozoic to Quaternary, overlying a

Precambrian cratonic basement [22–25]. The strati-

graphic column (Fig. 2) can be divided into a pre-

Cretaceous series [1], which is unconformably over-

lain by a continental to shallow marine Cretaceous

sedimentary cycle [22] and by a Cenozoic foreland

molassic and shallow marine cover [28].

The pre-Cretaceous series comprises Paleozoic

marine sediments, Triassic and Lower Jurassic marine

to continental rift deposits, and Late Jurassic back-arc

volcanoclastic sediments [1,23]. The tectonic setting

was dominated by Late Triassic–Lower Jurassic rift-

ing [26], induced by the E–W trending Tethyan sys-

tem [29], followed by an Upper Jurassic back-arc

extensional regime initiated by the onset of Andean

subduction [1] and associated activity of the continen-

tal Misahualli–Colan volcanic arc [30]. At c 140–

,

120 Ma [31], a major change in the geological

setting of the Oriente Basin occurred, with subduc-

tion and active arc magmatism ceasing, interpreted

as the result of the accretion of allochthonous ter-

ranes onto the Ecuadorian and Colombian margin

[29,31–34].

After a major sedimentary hiatus (c 120–110

Ma [21]), the Cretaceous sedimentary series was

deposited. It comprises fluvial to shallow marine

Aptian to Campanian deposits of the Hollın and

Napo Formations [21,22,25,35,36]. The Hollın-Napo

megasequence is characterized by cyclic sequences

of limestones, shales and sandstones. Its deposition

and distribution on a stable platform along a NW–SE

depocentre were controlled by worldwide eustatic sea

level fluctuations during Cretaceous times [22,27].


Finally, the Late Cretaceous–Cenozoic sedimentary

series was deposited after a major sedimentary hiatus

at the base of the Maastrichtian–lower Paleocene

sandstones and red beds of the Tena Formation, an

erosive event that may reflect the accretion of the

allochthonous Pallatanga, Macuchi Terranes and

coastal Pinon block [31,37–40]. This Late Cretac-

eous–Cenozoic cover represents the detrital section

associated with the development of the true Andean

foreland system [28].

Structurally, the Oriente Basin is characterized by

overprinted structures [41]. Pre-Cretaceous exten-

Vista

Pungarayacu

Yuralpa

Dayuno

Shu

Puma AucaOso

Jaguar

CononacoArmadillo

Villano-1

RIO PASTAZA

COLOMBIA

AN

DE

AN

CO

RD

ILLE

RA

1

1

0

2

78 77

La

Jivino

Central Corridor(Pre-existing Triassic-LowerJurassic rift structural limits)

Waponi

93±3.4 Ma(b)

82.2±2 Ma(b)

84±2 Ma(b)

91.2±

102.4±2.4 Ma(b)

106±5 Ma(a)

110±5 Ma(a)

RIO NAPO

93.±

Fig. 3. Geographic distribution and location of Cretaceous extrusive a

radiometrically analyzed sample sites are shown. Samples level with pane

sional systems, inherited from the NNE–SSW paleo-

rift structures [26], have been tectonically inverted

along three major oblique NNE–SSW right-lateral

transpressive wrench-fault zones (Fig. 1), which

have deformed the foreland basin system since the

Late Cretaceous [20]. From west to east, these tectonic

domains are: (i) the northern Sub-Andean system

(Napo Uplift), formed by an echelon NNW–SSE

positive flower structure, which was mainly active

during the Pliocene and Quaternary, but currently

still exhibits strong seismic and volcanic activity; (ii)

the Central Corridor, developed in Late Cretaceous

Ginta

shufindi

RIO AGUARICO

RIO NAPO

76

guna

Tapi

4.6 Ma(a)

0 10 20 30 40 50

KILOMETERS

EXTRUSIVE EVENTSbasaltic tuff cones

INTRUSIVE EVENTSgabbroic sills

Oil Wells

Sample Sites

Jaguar Seismic Section(Fig. 4)

main wrench zones

Radiometric ages from unpublished sources

3.4 Ma(b)

nd intrusive events along the Oriente Basin. Geochemically and

l (b) are 40Ar–39Ar and with panel (a) are 40Ar–40K.

Fig. 4. Reflection seismic sections showing the emplacement of extrusive magmatic rocks facies in the Ecuadorian Oriente Basin during

deposition of the Campanian upper Napo Formation. Interpretation is based on well and seismic data correlations. Reflection seismic data show

the structural control of pre-existing extensional features on the emplacement of alkaline eruptive sites: (a) Jaguar extrusive structure (tuff cone).

(b) Puma structure (tuff cones). See Fig. 3 for the location of seismic profiles.


Table 1

Summary of the extrusive and intrusive events recognized within the Cretaceous section of the Oriente Basin and found at different exploratory oil wells

Extrusive events

Basalticglassy tuffs

Intrusive events

Thickness (m) (**)

Biostratigraphicage

Stratigraphicrecord

Area (*)

NNE TapiVista Jivino LagunaIndillana−ItayaAucaArmadilloCononacoPumaJaguarPungarayacuWaponiYuralpaDayunoWSW

200−250180−200

3015−8040−8050−703090−13510

8070

Hollin FmUpper Hollin−Basal Napo

Lower−NapoLower−NapoUpper Napo FmUpper Napo FmUpper Napo FmUpper Napo FmUpper Napo FmUpper Napo Fm

Upper Napo FmUpper Napo Fm

Middle AlbianMiddle Albian

Upper Alb−lower Cenom.Upper Alb−lower Cenom. Turonian−Santonian Turonian−Santonian Turonian−Santonian Coniacian−SantonianConiacian−SantonianSantonian−Campanian

Santonian−CampanianSantonian−Campanian

Basalticdikes

Gabbroic.sills

Thickness(m) (**)

Radiometricage (Ma)

Radiometricmethod

1−2 1−2 1−25 50−60 20 0.1−1

50150 5−190 50

110.2 ± 5.2 106 ± 5 102.4 ± 2.4 92 ± 3.9

93.1 ± 0.7; 91.2 ± 4.6

93.0 ± 3.4 82.2 ± 2.0 84 ± 2

40Ar/40K (***)40Ar/40K (***)40Ar/39Ar40Ar/40K (***)

40Ar/39Ar;40Ar/40K (***)

40Ar/39Ar40Ar/39Ar40Ar/39Ar (***)

�

Biostratigraphic information based on [21,22]. New 40Ar–39Ar data are shown in Tables 3a and 3b. Bold ages in the right column indicate 40Ar–39Ar plateau ages determined in this

study.

(*) Related to exploratory oil wells along the Oriente Basin of Ecuador (source: Petroproduccion-Geolab).

(**) Apparent thickness defined from well log analysis and the associated seismic section.

(***) Unpublished radiometric age data (source: Petroproduccion Library facilities). Age determinations on whole rock samples were performed by request of Texaco Petroleum

Company at the Geochron Laboratories Division, Krueger Enterprises, Inc.

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Table 2

Representative analyses of major and trace elements by XRF. REE, Th, Ta, Hf, Sc, U and Cs were analyzed by ICP-MS

Jivino-1

DJGJ1

Auca-16

DJGA16

Auca-23

DJGA23

Auca-20

A FLD

Pungarayacu-9

DJGP9

Pungarayacu-16

P9BX16

Pungarayacu-39

P9BX39

Yuralpa-1

DJGY1

Waponi-1

RBTW1

Yuralpa-2

RBTY2

Pungarayacu-10

PUNGA

Yur

Centro-1

AL C1

Dayuno-1

ONI 2

SiO2 (wt.%) 45.62 43.02 43.47 43.25 42.31 42.04 41.83 46.29 41.69 46.22 43.01 45.23 41.70

Al2O3 11.28 12.58 9.51 12.33 10.85 10.31 10.91 9.53 9.22 12.76 10.75 9.14 9.42

TiO2 3.03 3.05 2.61 3.06 3.15 2.89 3.11 2.54 3.74 3.29 3.06 2.45 3.73

FeO 11.52 13.02 11.48 14.12 11.97 11.85 13.17 12.43 12.42 11.38 12.52 12.25 12.88

MnO 0.17 0.11 0.13 0.12 0.19 0.18 0.20 0.18 0.20 0.16 0.18 0.18 0.21

CaO 11.86 10.31 11.81 10.31 12.22 11.78 11.80 11.10 11.82 10.69 12.48 11.07 12.37

MgO 10.30 8.01 13.63 8.24 12.37 14.56 12.54 15.86 15.47 9.27 12.84 15.54 14.41

K2O 1.27 1.91 0.77 1.90 1.17 1.20 1.54 1.09 0.23 1.61 0.96 1.08 0.23

Na2O 2.62 2.42 1.85 2.19 3.13 2.98 2.64 2.57 2.14 2.85 2.77 2.56 2.66

P2O5 0.58 0.79 0.73 0.82 0.84 0.78 0.83 0.44 0.97 0.70 0.82 0.47 0.97

LOI 4.19 8.75 7.12 8.88 2.48 1.77 1.95 0.46 5.93 2.32 3.70 0.65 4.33

Total 102.44 103.96 103.11 105.22 100.68 100.35 100.52 102.49 103.83 101.26 103.09 100.60 102.90

Ni (ppm) 258.00 58.00 344.00 75.00 234.00 409.26 264.66 404.00 329.00 178.00 323.00 404.00 334.00

Cr 317.00 277.00 482.00 304.00 314.00 550.30 348.78 535.00 520.00 256.00 410.00 557.00 524.00

Sc 24.00 22.50 21.20 21.70 28.00 24.90 27.20 32.50 30.60 23.90 19.00 24.10 22.60

V 264.00 276.00 250.00 266.00 297.00 242.33 263.82 246.00 286.00 264.00 272.00 225.00 279.00

Rb 26.00 38.00 16.00 35.53 14.00 16.53 21.59 17.00 6.43 29.13 12.16 19.01 4.73

Sr 662.00 1008.00 460.00 916.66 782.00 873.67 740.36 383.00 726.12 740.55 860.44 519.80 706.27

Ba 518.00 697.00 380.00 605.38 559.00 478.02 507.61 301.00 621.77 460.91 514.21 317.22 601.66

Nb 60.50 84.00 60.00 73.76 81.00 76.29 79.11 93.28 100.04 62.71 73.78 40.04 99.44

Zr 194.00 246.00 187.00 223.17 233.00 207.95 219.38 168.00 290.15 252.50 209.41 150.74 281.61

Y 25.00 26.00 23.00 26.10 26.00 24.87 26.79 32.41 29.90 28.28 24.85 18.92 26.31

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La 36.93 47.57 40.77 48.49 47.08 45.91 47.51 57.51 57.13 41.61 46.95 26.60 57.26

Ce 70.41 88.99 77.11 90.03 89.20 84.47 88.16 107.03 107.72 79.56 86.61 51.32 107.15

Nd 34.85 41.64 38.49 42.78 43.39 40.22 42.28 52.46 51.50 38.99 41.32 26.77 51.70

Sm 8.16 9.65 8.60 9.81 9.67 9.24 9.76 12.25 11.62 9.22 9.52 6.69 11.49

Eu 2.73 3.11 2.81 3.16 3.20 2.97 3.12 3.97 3.61 2.99 3.04 2.20 3.62

Gd 7.61 8.12 7.49 8.49 8.55 8.20 8.60 10.53 9.98 8.27 8.45 6.07 9.66

Dy 5.90 6.12 5.57 6.12 6.36 5.84 6.23 7.69 6.89 6.28 5.98 4.52 6.42

Ho 1.04 1.06 0.95 1.05 1.06 0.98 1.05 1.32 1.16 1.09 1.01 0.77 1.05

Er 2.34 2.43 2.15 2.45 2.43 2.20 2.32 2.84 2.64 2.57 2.29 1.76 2.37

Tm 0.31 0.31 0.27 0.30 0.30 0.27 0.29 0.34 0.33 0.33 0.28 0.22 0.29

Tb 1.18 1.12 1.19 1.49 1.36 1.19 1.16 0.85 1.26

Yb 1.73 1.70 1.49 1.69 1.69 1.47 1.56 1.83 1.76 1.83 1.53 1.21 1.53

Lu 0.25 0.24 0.22 0.24 0.23 0.21 0.22 0.25 0.25 0.27 0.21 0.17 0.21

Hf 4.73 5.34 4.61 5.30 5.52 4.95 5.23 6.92 6.84 6.11 5.22 3.99 7.02

Ta 3.55 4.50 3.47 4.38 4.86 4.60 4.75 5.82 6.46 3.91 4.55 2.58 6.46

Pb 2.29 3.78 5.59 4.49 3.46 2.45 4.59 2.23 4.06 4.04 4.94 2.78 3.77

Pr 8.33 10.21 9.11 10.32 10.44 9.62 10.04 12.51 12.31 9.19 9.91 6.15 12.39

Th 3.57 4.87 3.81 5.59 4.81 5.71 5.89 7.10 7.65 5.64 5.40 2.89 6.65

U 1.10 1.57 1.21 1.58 1.64 1.62 1.71 1.58 2.19 1.51 1.55 0.82 1.92

Mg/Mg+Fe 66.00 57.00 72.00 56.00 69.00 72.50 67.00 74.00 73.00 64.00 69.00 73.00 71.0087Sr/86Sr 0.70519 0.70442 0.7035587Sr/86Sr (i) 0.70503 0.70428 0.70345143Nd/144Nd 0.51282 0.51284 0.51282143Nd/144Nd (i) 0.51272 0.51275 0.51274

Representative samples of diabasic dikes and gabbroic sills come from well cores and cuttings (see Fig. 3 for location). The Mg num er was calculated assuming FeO/Fe2O3=0.33.

FeO* is total iron as FeO. Sr and Nd isotopic ratios of three samples were calculated using international standards [43]. 87 Sr/86 Sr (i nd 143 Nd/144 Nd (i) are age corrected isotopic

data for approximately 100 Ma.

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b

) a


and Cenozoic times, resulting from the inversion of

the NNE–SSW Upper Triassic to Lower Jurassic rift,

which emerges and crops out in the Southern Sub-

Andean foothills [26]; (iii) the Eastern Inverted Sys-

tem, generated by the inversion of Upper Jurassic half

grabens, which developed along the eastern border of

the Oriente Basin [20].

3. Characteristics and regional distribution of the

Oriente Basin basaltic volcanism (OBB)

The regional distribution of the Cretaceous extru-

sive and intrusive bodies recognized within the Hol-

lin-Napo sedimentary infill of the Ecuadorian Oriente

Basin is shown in Fig. 3. Although the volume of

eruptive products is relatively small, the OBB is a

NNE-trending system made up of at least 30 isolated

eruptive centers. They comprise tuff cones or mono-

genetic volcanic fields mainly characterized mostly by

hyaloclastites, the largest being only 200–250 m thick

and covering an area of 2–3 km2 (e.g., Fig. 4—Jaguar

and Puma areas, see also Fig. 3 for location). In

addition, several basaltic dykes, 1–10 m wide, and

at least four major shallow intrusive fine-grained gab-

broic to diabasic sills have been identified, the largest

being 80–200 m thick and covering an area of 15–20

Km2 (e.g., Yuralpa–Dayuno and Laguna areas, see

Fig. 3 for location).

The extrusive facies are mainly characterized by

tuff cones and tuff rings interpreted on several seismic

cross sections (Fig. 4) and corroborated by several

drill core samples, well logs and outcrops (Table 1,

see Fig. 3 for sample locations). Volcanoclastic com-

ponents of the volcanic piles consist of stratified, thin-

bedded, basaltic tuff units of well-to-poorly-sorted

fine ash. The fragments consist mainly of hypocrystal-

line basalt (hyaloclastites). Accretionary lapilli are

also very common, indicative of sub-aqueous deposi-

tional conditions. Much of the basaltic glass is altered

to palagonite. The volcanoclastic deposits which are

mainly altered tuffs and palagonitized hyaloclastites,

recognized at different locations within the shallow

marine Cretaceous section [21,22], suggest that the

Cretaceous extrusive bodies in the Oriente Basin

resulted from hydrovolcanic eruptions with a charac-

teristic Surtseyan eruptive style [42]. No basaltic lavas

have been identified.

The time-equivalent intrusive igneous bodies are

characterized by alkalic fine-grained gabbroic to dia-

basic sills and basaltic dykes, identified at different

locations from well-preserved cores and cutting sam-

ples (Table 1, see Fig. 3 for location). They were

emplaced stratigraphically at various horizons within

the Cretaceous sedimentary series. Despite the vari-

able emplacement, there remains a strong composi-

tional and mineralogical uniformity among the

gabbroic to diabasic sills and dikes. Petrographically,

the alkaline sills show a fine-grained phaneritic tex-

ture, and are primarily composed of ophitically inter-

grown olivine, labradorite and augite phenocrysts.

The diabase dykes typically contain olivine as the

dominant phenocryst phase with subordinate labrador-

ite and clinopyroxene. The intergranular groundmass

contains plagioclase microlites and granules of clin-

opyroxene, olivine and magnetite.

4. Analytical procedures

The abundance of major and trace elements in the

alkaline magmas from the Cretaceous Oriente Basin

was determined from 12, well-preserved, core samples

(Table 2). The samples are dykes or shallow intrusive

sills that microscopically do not show significant

alteration to palagonite, if any.

A relatively high LOI of some samples, with values

of up to 8 wt.%, appears to demonstrate a certain

degree of alteration. Nonetheless, based on the Che-

mical Index of Weathering (CIW [44]), the Chemical

Index of Alteration (CIA [45]), and certain elemental

ratios such as Th/Ta, La/Ta, La/Nb or Ba/Zr besides

many others, lead to the conclusion that even samples

with high LOI do not shift out of the general scheme

demonstrated by samples with usually low LOI which

are considered to be fresh or unaltered of the same

sample set. In particular, CIW values of 28–37 and

CIA values of 27–36, fall within known fields of

unaltered Mesozoic Basalts [46].

Fig. 3 shows the location of these samples and their

characteristics are listed in Table 1. The distribution of

the OBB was determined by reflection seismic sec-

tions provided by the Ecuadorian National Oil Com-

pany (Petroproduccion). However, the thick overlying

Cenozoic section severely limits the amount of Cre-

taceous sedimentary outcrops in the Oriente Basin,

A) Jivino-1 whole rock

B) WAPONI-1 whole rock

C) Yuralpa-1 whole rock

0

30

60

90

120

150

180

210

240

0 10 20 30 40 50 60 70 80 90 100

Age

(M

a)

MSWD = 1.93

102.41 ± 2.37 Ma

0.0000

0.0010

0.0020

0.0030

0.0040

0.000 0.008 0.016 0.024 0.032

36A

r / 4

0Ar 101.70 ± 2.41 Ma

MSWD = 0.18

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60 70 80 90 100

Age

(M

a)

93.02 ± 3.42 Ma

MSWD = 0.940.0000

0.0010

0.0020

0.0030

0.0040

0.000 0.008 0.016 0.024 0.032

36A

r / 4

0Ar 91.44 ± 3.87 Ma

MSWD = 0.50

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60 70 80 90 100

Cumulative 39Ar Released (%)

Age

(M

a)

82.15 ± 1.99 Ma

MSWD = 7.030.0000

0.0010

0.0020

0.0030

0.0040

0.000 0.010 0.020 0.030 0.040

39Ar / 40Ar

36A

r / 4

0Ar 81.47 ± 2.60 Ma

MSWD = 0.00

0.00.00.20.20.40.40.60.60.80.81.01.0

K/C

a

0.00

0.10

0.20

0.30

0.40

K/C

a

0.00.40.81.21.62.0

K/C

a

Fig. 5. Representative age and K/Ca spectra (left) and inverse isochron (right) plots for 40Ar–39Ar incremental heating experiments, performed at

Oregon State University on diabasic dikes (Waponi-1 and Jivino-1) and a gabbroic sill (Yuralpa-1) in drilled, whole rock, core samples of the

OBB. The vertical range of horizontal boxes indicates the estimated analytical error (F2j) for each step age. A plateau age (indicated) has been

determined from the weighted mean of contiguous, concordant step-ages. The 36Ar/40Ar vs. 39Ar/40Ar isotope correlation diagrams are

constructed from the step Ar-compositions measured. The isochron age (indicated) is calculated from the best-fitting line through collinear step

compositions. Tables 3a and 3b present full isotopic data for the new 40Ar/39Ar plateau and isochron ages from the Oriente Basin Basaltic

Volcanism (OBB). Analytical procedures are described in [47,50].



concealing the true amount of magmatism generated

during the OBB igneous event.

Major-elements and Ni, Cr, Sc, V, Ba, Rb, Sr,

Zr, Y and Nb were analyzed by XRF procedures in

the Geoanalytical Laboratory at Washington State

University (Johnson et al., pers. comm., 1998).

The precision of the data and sample heterogeneity

were tested by multiple analyses of a single speci-

men. Major-element precision is b 2% of the abso-

lute abundance; trace-element precision is b 5%

except for Nb and Rb, which are 10% in low-abun-

dance samples. Rare-earth elements (REE) were ana-

lyzed by standard Inductive by Coupled Plasma Mass

Spectrometry (ICP-MS) techniques, at Washington

State University. In addition, Sr and Nd isotopic

data, listed in Table 2, were determined at Cornell

University using a VG Sector Mass Spectrometry.

They were calculated using the NBS 987 Sr-isotope

standard (Mean: 0.7102452F0.000023; 2j S.D.:

2.30391E-05) and the Ames Nd-standard (Mean:

0.51213433F0.000008; 2j S.D.: 8.3267 E-06).

Further procedural details can be found in [43]. In

Table 3a40Ar–39Ar incremental heating results for sills from the Oriente Basin (Ec

Sample, material Heating step

(8C)

36Ar 37Ar 39A

Yuralpa-1, whole rock 550 0.00035 0.02881 0.07

( J =0.001651) 750 0.00121 0.95230 0.17

950 0.00100 0.05468 0.21

1150 0.00011 0.07595 0.18

1250 0.00001 0.98607 0.16

1300 0.00007 0.57876 0.01

Weighted plateau age (steps 2–5): 82.15F1.99

Waponi-1, whole rock 550 0.00783 0.02832 0.00

( J =0.001440) 750 0.00209 0.01204 0.00

950 0.00315 0.01639 0.00

1150 0.00055 0.01382 0.00

1250 0.00031 0.04887 0.00

1300 0.00016 0.14883 0.00

1400 0.00026 0.65012 0.00


Jivino-1, whole rock 550 0.00044 0.03429 0.01

( J =0.001534) 750 0.00167 0.10707 0.09

950 0.00270 0.14306 0.18

1150 0.00007 0.08652 0.15

1250 0.00014 0.12646 0.11

1300 0.00021 0.96509 0.08


37Ar has been corrected for decay since irradiation. K/Ca ratios are d

addition, the 87Sr/86Sr (i) and 143Nd/144Nd (i)

values (Table 2) are age corrected isotopic data for

approximately 100 Ma.40Ar/39Ar incremental heating experiments were

performed for five drill core gabbroic to diabasic

samples from the Oriente Basin (Figs. 3 and 5; Tables

3a and 3b). Age determinations on whole rock sam-

ples were performed at Oregon State University using

standard 40Ar–39Ar incremental heating techniques

[47]. Because of the aphyric nature of the samples,

no preparation other than coring, cutting and washing

~ 100 mg mini-disks of rock from hand specimens

was performed prior to neutron irradiation in the OSU

TRIGA reactor for 6 h at 1 MW power. Individual

ages for each 40Ar–39Ar temperature step were calcu-

lated (using the ArArCALC software, [48]) after cor-

rection for background, mass discrimination, isotopic

interference and atmospheric argon content. All ages

are calibrated to the FCT-3 biotite standard (28.04 Ma,

[49]) and uncertainties are 2 S.D., based on errors in

both peak height fitting and the interpolation of neu-

tron fluence measurements ( J-values). Age spectra

uador)

r 40Ar

(r)

AgeF2s

(Ma)

%40

Ar (r)

%39

Ar

K/Ca

585 2.08924 80.23F1.16 95.3 9.2 1.132

775 5.18536 84.86F2.11 93.5 21.5 0.803

133 6.08387 83.77F0.93 95.3 25.6 1.662

560 5.21339 81.78F1.12 99.3 22.5 1.051

004 4.50298 81.92F0.33 99.9 19.4 0.070

561 0.30627 57.50F6.79 93.6 1.9 0.012

MSWD: 7.03

501 0.20698 104.22F19.59 8.2 10.6 0.076

820 0.29889 92.33F5.96 32.7 17.3 0.293

862 0.34621 101.41F8.75 27.1 18.2 0.226

589 0.21375 91.84F8.09 56.6 12.4 0.183

673 0.24391 91.77F6.42 72.4 14.2 0.059

534 0.19296 91.56F8.03 80.7 11.2 0.015

769 0.27829 91.70F5.29 78.3 16.2 0.005

MSWD: 0.94

956 0.79728 109.42F5.52 86.0 3.0 0.245

928 3.80170 102.98F0.85 88.5 15.0 0.399

253 6.97685 102.79F1.05 89.7 27.6 0.549

655 5.90993 101.56F0.94 99.7 23.7 0.778

576 4.39505 102.12F1.21 99.0 17.5 0.394

736 5.33395 161.51F1.74 98.9 13.2 0.039

7 MSWD: 1.93

etermined from measured 39Ar/ 37Ar. 40Ar (r) is radiogenic 40Ar.

Table 3b

Summary of radiometric dating results for five samples from the Oriente Basin basalts

Sample, material Total

fusion age

(Ma)

2jerror

Plateau

age

(Ma)

2jerror

N MSWD Isochron

age

(Ma)

2j MSWD 40Ar/36Ar

initial

2j error J

Auca-16, whole rock 165.63 3.64 None

developed

None

developed

0.001477

Pungarayacu,

whole rock

231.7 4.98 None

developed

None

developed

0.001402

Yuralpa-1, whole rock 82.38 1.93 82.15 1.99 4/6 7.03 81.47 2.60 0.002 469.0 166.9 0.001651

Waponi-1, whole rock 94.91 3.96 93.02 3.42 7/7 0.94 91.44 3.87 0.50 299.8 4.86 0.001440

Jivino-1, whole rock 110.48 2.50 102.41 2.37 4/6 1.93 101.7 2.41 0.18 328.8 28.8 0.001534


(age vs. % 39Ar released) and 36Ar/40Ar vs. 39Ar/40Ar

isotope correlation (isochron) diagrams are shown in

Fig. 5. Plateau ages were calculated where appropri-

ate, using the procedure described in [47,50].

Two samples produced ideal gas release patterns,

from which reliable crystallization ages can be calcu-

lated. The Jivino-1 sample exhibits a 4-step plateau

with a weighted (by 1/variance) mean age of

102.4F2.4 Ma. The corresponding isochron age is

concordant, at 101.5F2.4 Ma and yields a 40Ar/36Ar

intercept (329F29) very close to the atmospheric

value (296). The statistical measure of significance,

MSWD or mean square of weighted deviations, is

appropriately low (below the ~ 2.5 critical value)

that the plateau age can be accepted as a reliable

estimate of the crystallization age. Similarly, a 7-step

plateau age of 93.0F3.4 Ma for the Waponi-1 sample

passes all tests of reliability as a crystallization age.

The Yuralpa-1 sample produced a pattern that approx-

imates a plateau at 82.2F2.0 Ma, but with a slightly

decreasing age with increasing temperature trend that

reflects 39Ar recoil within the sample during irradia-

tion. In this case, the MSWD is larger than the critical

value, indicating dispersion of the step ages beyond

that expected from the step age errors. Because the

near-concordant trend in the central four step ages

accounts for 89% of the total gas released, the calcu-

lated age is probably close to the crystallization age;

however, we cannot discount some Ar-loss. Reliable

crystallization ages were determined despite signifi-

cant groundmass alteration (smectitic clay) in Jivino-1

and Waponi-1, indicated by LOI of 4–6%. We con-

clude that our standard 400 8C pre-heating was suc-

cessful in removing much of the loosely-held Ar from

non-retentive alteration phases. The lowest tempera-

ture step in each of these experiments reveals the

effect of greater proportions of atmospheric Ar (larger

age uncertainties) and Ar-loss (sample Yuralpa-1).

The remaining two samples contained significant

amounts of excess Ar (undegassed at cooling) that pro-

duced old and variable step ages. The isochron plots for

these samples do not reveal any common component of

initial Ar than can be subtracted from the step ages,

hence the only useful age estimates are the ages of the

youngest steps (least excess Ar), which are 93.1F0.7

Ma (Auca-16) and 123.8F0.9 Ma (Pungarayacu). We

note that these are maximum age estimates only.

Finally, other 40Ar/39Ar and 40Ar/39K radiometric

ages were obtained from unpublished sources (Petro-

produccion National Oil Company, pers. comm.,

1998). The values are shown in Table 1 (see Fig. 3

for location) for comparison purposes.

5. Geochemical character and petrogenesis

The composition and abundance of major and trace

elements from representative rock samples of the Cre-

taceous OBB are summarized in Table 2. In spite of

their different geographic location and age distribution

(110–80 Ma), they exhibit a restricted range of com-

positional variation, both in major and trace elements.

On a total alkali vs. silica plot (Fig. 6), the OBB

igneous rocks fall into the alkaline basaltic field.

They are also characterized by high contents of TiO2

(2.5–3.7 wt.%), K2O (0.23–1.91 wt.%), Na2O (1.85–

3.13 wt.%) and P2O5 (0.444–0.967 wt.%), with most

values being greater than 1.0, 1.2, 2.4 and 0.7 wt.%,

respectively. They yield a high abundance of MgO

(8.01–15.8 wt.%), Ni (58–404 ppm) and Cr (277–557

ppm) suggesting a primitive nature. Mg numbers

(100�Mg/(Mg+Fe2+)) range from 56 to 73 and

1

2

3

4

5

41 42 43 44 45 46 47 48 49 50 51 52 53 54

MORB

Alkalin

e

Tholeite

SiO2 (wt%)

K2O

+N

aO

(%

wt)

OBB

Fig. 6. Plot of Na2O+K2O vs. SiO2 of 13 samples from the OBB,

showing the divide between alkaline and tholeiitic basalts [51]. Mid-

ocean ridge basalt (MORB) composition from [51].

0.1

1

10

100

1000

Rb Ba Th U Ta Nb K La Ce Sr N

OBB Galapa

Antartica Pen. San Q

Ro

ck/M

OR

B

0.1

1

10

100

1000

Rb BaTh U Ta Nb K La Ce Sr Nd P

Ro

ck/M

OR

B

Fig. 7. MORB normalized incompatible trace element distribution from the

variation, and (b) a comparison of the OBB rocks (representative sample) w

alkaline basalts (CAB). Data sources are shown in Tables 3a and 3b. MO


most values are greater than 64 (Table 2). These

primitive compositions suggest the samples crystal-

lized from a magma, which had experienced minor

fractionation since its origin in its mantle source

region, assuming a typical Mg number of 88–90 for

the mantle [52].

The range of trace-element compositions for the

Cretaceous OBB relative to average mid-ocean ridge

basalts (MORB) is shown in Fig. 7a. With the excep-

tion of two samples (RBTW-1 and ONI-2) that show

low contents of K and Rb, all the incompatible ele-

ment concentrations in the OBB lavas are elevated in

comparison with normal MORB. However, K/Rb

d P Sm Zr Hf Eu Ti Tb Y Yb Lu

gos (OIB) Patagonia (CAB)uintin Reventador (CALC-ALKALINE)

SmZr Hf Eu Ti GdTbDy Y Er TmYbLu

P9BX16

P9BX39

DJGJ1 DJGA16 DJGA23DJGP9 DJGY1 RBTW1RBTY2 PUNGAALC1 ONI2 AFLD

Cretaceous OBB, showing (a) the restricted range in compositional

ith selected oceanic island basalt (OIB), calc-alkaline and continental

RB average data are taken from [53].

10 204 300

200

400

600

MORB

OIB

Ba/La=10

Ba/La=20

continentalmagmatic arc

La/Ta

Ba/

Ta

OBB

Fig. 8. (a) Plot of Ba/Ta vs. La/Ta ratios (modified from [14]

showing the similarity between OBB magmas and ocean island

basalts (field labelled OIB). The field for Andean continental mag

matic arcs is also shown [5].


ratios in all the OBB samples are relatively constant

(~ 500). Table 4 shows selected incompatible ratios

for the Cretaceous OBB rocks and from several other

basaltic tectonic settings. Overall, the Cretaceous

OBB samples show trace-element abundances similar

to alkaline magmas from several continental (slab

window-related basalts) and intra-oceanic settings

(OIB), with low large-ion-lithophile element (LILE)/

high-field-strength element ratios (HFSE) (e.g., Ba/

Nbc6.33–8.56; Th/Tac1.00–1.44; La/Tac8.84–

11.75; La/Nbc0.57–0.67; Ba/Zrc1.79–2.83).

For comparison, primitive-mantle normalized

incompatible element abundance patterns for alkaline

basalts from San Quintin, Baja California [55], basalts

from the Antarctic Peninsula [8,9] and Patagonia

[6,7], and for oceanic island basalts from Galapagos

Islands [51,56] are given in Fig. 7b. In general, the

OBB rocks exhibit similar trace element abundance

patterns to OIB and some continental basalts. Low Th/

Ta (0.98–1.44), La/Ta (8.84–11.75) and Ba/Ta (52–

155) ratios are typical of intra-plate alkaline magmas

that have not interacted with enriched lithosphere and/

or continental crust (Fig. 8), as is suggested for the

Antarctic Peninsula basalts [8]. The high absolute

HFSE abundance in the OBB lavas, coupled with

the lack of depletion of Nb and Ta that is characteristic

of subduction-related magmas [5], corroborate this

observation. For comparison, Fig. 7b shows typical pri-

Table 4

Selected incompatible-element ratios for basaltic rocks of the Cretaceous Oriente Basin and for basalts from several other tectonic settings

OBB Subduction-related basalts MORB OIB CAB

Oriente Basin IAT HAB and CA Mantle plume origin Slab window Roll-back

San Quintin Cameroon (Antarctic Penins) James Ross Island

K/Zr 35.5–67.8 147 216 12 44 57.5–60.08

Rb/Zr 0.06–0.15 0.21 0.35 0.01 0.1 0.128–0.138 0.129 0.038–0.119 0.09–0.125

Ba/Zr 1.79–2.83 5 7.5 0.1 1.7 1.54–1.83 1.93 0.399–1.188 0.877–1.43

Ba/Nb 6.33–8.56 157 214 4 7 7.14–9.19 8.09 2.69–8.21 4.73–6.14

Ba/Ce 4.93–7.83 30 13 1 5 5.52–6.86 5.25 1.81–5.4 7.16–8.52

La/Nb 0.57–0.67 1.86 7.14 0.97 0.66 0.66–0.74 0.8056 0.61–0.881 1.22–3.47

Zr/Nb 2.63–3.89 31 29 27 4 4.5–5.37 4.194 4.657–7.66 5.87–10.6

Zr/Y 7.76–9.46 1.8 2.7 2.9 7.3 9.64–7.807 10.34 5.43–18.57 4.81–10.46

Ce/Yb 40–60 – – – – 24.61–35.29 – 15.67–44.978 10.97–45.33

Sm/Yb 4.71–6.86 – – – – 2.69–3.72 – 2.606–4.387 2.61–4.6

CeN/YN 6.51–8.35 1.2 3.5 0.7 4.73–9.1 4.88–7.825 8.78 2.58–9.15

OBB—Oriente Basin basalts (Fig. 3). Subduction-related basalts: IAT—island-arc tholeiite; HAB—high-alumina basalt; CA—calc-alkaline

basalt (data source: [54,57]). OIB—ocean island basalt. Data from the Galapagos Islands ([51,56] CAB—continental alkaline basalts; mantle

plume origin. Data from the Cameron line [58], San Quintin [4,55]; slab window-Antarctic Peninsula [8,9], Southern Patagonia [5–7]; Roll

back–James Ross Island [11]).

)

-

mitive-mantle normalized incompatible element abun-

dance patterns of a calc-alkaline lava (Reventador

volcano) from the sub-Andean zone of Ecuador [57].

The relatively consistent abundances of the HREE,

the typical profile of alkali basalts and the geochemical

signature of this alkaline magma (LaN/YbNc15–27,

Sr/Yc12.4–32.9, Ce/Ybc40.7–62.3 and Sm/Ybc4.7–6.9) indicate low degrees of partial melting from a

predominantly asthenosphericmantle source region, and

suggest that residual garnet was present during partial

melting yielding unevolved basalts [58]. Such a garnet-

bearing mantle source requires melt generation at high

pressure, typically at depths of z 60–80 km [59].

-


Sr and Nd isotopic ratios of three OBB samples are

shown in Table 2. The OBB magmas exhibit values

within the range of those observed for the OIB main

array [9]. They show a wide range of 87Sr/86Sr values

(0.703450–0.705027) and restricted 143Nd/144Nd

(0.512724–0.512751) ratios, similar to values yielded

by cratonic continental alkali basalts from Patagonia

[6] and close to those fields occupied by OIB basalts

(e.g., Azores Island [4]) and the James Ross Island–

Antarctic Peninsula [11].

6. Geochronology: timing of the OBB

Three new 40Ar/39Ar radiometric ages from drill

core basaltic samples combined with unpublished40Ar/39K and 40Ar/39Ar radiometric ages from differ-

ent locations within the Oriente Basin are summarized

in Tables 1 and 3b (locations shown in Fig. 3). These

ages, combined with biostratigraphic ages of the sur-

rounding Hollin and Napo sedimentary rocks [e.g.,

20,21,23,25,34,35], indicate a systematic age progres-

sion of the mafic magmas with geographic position

for the emplacement from Albian through Campanian

times (i110–80 Ma) (Table 1). However, no evidence

of basaltic volcanism has been found in younger

Tertiary sediments.

The oldest volcanic episode, a basaltic tuff cone,

was emplaced in the Middle Albian Hollin Formation

within the north–central part of the Oriente Basin

(Vista and Tapi areas, Fig. 3). Radiometric ages of

diabase dikes intersected by the Tapi-5 and Vista-1

exploratory wells (40Ar/39Ki110.2F5.2 Ma and

106F5 Ma, respectively [27]), corroborate the bios-

tratigraphic record at these locations.

Evidence of diachronous volcanism is found dur-

ing deposition of the Albian–Campanian Napo Fm.

with volcanism advancing towards the SSW. Thus,

alkaline basaltic volcanism was coeval with sedimen-

tation of the upper Albian–Cenomanian lower Napo

section in the central part of the basin (Laguna and

Jivino areas), the Turonian middle Napo Fm (Auca-

Armadillo and Waponi areas) and the Campanian

upper Napo Fm in the western–central part of the

Oriente Basin (Yuralpa and Dayuno areas). Radio-

metric ages of gabbroic to diabasic sills and dikes

recovered from several exploratory wells at the corre-

sponding geographic locations confirm the biostrati-

graphic record for these volcanic events, respectively:40Ar/39Ari102.4F2.4 Ma (Jivino-1), 40Ar/39Ki91F4.6 Ma (Auca-16, Petroproduccion, pers.comm)

and 40Ar/39Ari93.1F0.7 Ma maximum age (see

above), 93.0F3.4 Ma (Waponi-1) in the central part

of the basin; 40Ar/39Ari84F2 Ma (Dayuno-1, Pet-

roproduccion, pers.comm.); and 82.2F2.0 Ma (Yur-

alpa Centro-1), in the western–central part of the basin

(Table 1, locations shown in Fig. 3).

Therefore, the systematic age progression from

Albian (~110 Ma), in the northern part of the Ecua-

dorian Oriente Basin, to lower Campanian (~82 Ma),

in the west central part of the Ecuadorian Oriente

Basin, suggests that magmatism migrated towards

the SSW along the active Central Corridor (Fig. 3).

7. Structural control of cretaceous basaltic

magmatism in the Oriente Basin

The geographic distribution of the Cretaceous alka-

line extrusive and intrusive bodies, in the Ecuadorian

Oriente Basin, is mainly restricted to the NNE–SSW

Central Corridor (Fig. 3). Reflection seismic data

evidence that major pre-Cretaceous extensional struc-

tures largely control the location and occurrence of

these alkaline eruptive sites, particularly along deep

NNE normal faults bordering Triassic–Jurassic gra-

bens and half-grabens (Fig. 4). Some of these normal

faults were reactivated as transpressive, right lateral

strike-slip structures related to major tectonic inver-

sion. Seismic evidence of some compressional synse-

dimentary deformation suggests that this regional

tectonic event occurred from Turonian to Maastrich-

tian times in the Oriente Basin [20,27]; however, it

started probably in Middle Albian, as it is evident in

the eastern Maranon Basin of Peru [60], where this

period is marked by compressional inverted struc-

tures. Nonetheless, it can be related to the Peruvian

contractional phase defined in the northern Central

Andes, corresponding to a period of high convergence

rate between the Farallon and the South American

plates [61]. The structural association with volcanism

suggests that pre-existing extensional structures

exerted fundamental control over the generation of

alkalic magmatism during the evolution of the Cretac-

eous Oriente Basin. Magma ascent was channeled by

pre-existing structural discontinuities, corresponding


to areas of previously thinned lithosphere that were

related to the Triassic–Jurassic rift system and Late

Jurassic back-arc extensional features. As suggested

by [15], these areas of continental weakness may

produce asthenospheric upwelling, which eventually

triggers partial melting.

During the Upper Cretaceous, magmatism

migrated to the SSW, along the Central Corridor of

the Oriente Basin, probably as a result of the lateral

propagation of the transpressive inversion of the

Triassic–Jurassic rift. Given the strong dextral

wrenching component of northern Andean deforma-

tion during this period and the NNW–SSE orientation

of the paleo-rift, the inversion was probably a strike-

slip dominant inversion with reactivation of NE–SW

to E–W extensional faults, which permitted ascent of

magma.

8. Discussion: relations between OBB and the

geodynamic evolution of north-western South

America

The Ecuadorian Oriente Basin records drastic geo-

dynamic changes associated with major tectonic plate

reorganization within the northwestern margin of

South America during Late Jurassic and Early Cretac-

eous times. This period was marked by significant

geodynamic changes, with the end of subduction

and the generation of the consequently active arc

magmatism (Misahualli–Colan arc) [29,30]. This

magmatic gap is considered a result of the oblique

accretion of oceanic and continental exotic terranes

onto the Ecuadorian and Colombian margin [29–34].

The eastern boundary of these accreted terranes is

marked by the occurrence of tectonic slides of HP

metamorphic rocks [34], which yield Early Cretac-

eous cooling ages (i.e., 132 Ma K/Ar age [62], 130–

115 Ma Ar/Ar [63,64]) and are thought to represent a

suture zone [33,38,40,62,65–67].

Independently of the nature of these allochthonous

terranes, during collisional events of continental and/

or oceanic fragments, the margin is uplifted and sub-

mitted to hiatus and/or erosions [68]. This may be a

consequence of the subduction blocking and tectonic

underplating of oceanic material beneath the continen-

tal margin [69]. Along the Oriente Basin, these pro-

cesses are clearly evident, resulting in important

paleogeographic changes [70]: uplift and erosion of

the pre-Cretaceous section, as shown by a major

sedimentary hiatus and regional unconformity at the

base of the Hollin Formation; installation of stable

platform conditions corresponding to the Cretaceous

shallow marine Oriente Basin system [21,22]; and

development of intra-continental alkali basaltic mag-

matism oriented parallel to the previous subduction

zone and located coincident to the major Triassic–

Jurassic rift system in the central part of the Basin

(Central Corridor [20]). Unfortunately, the geophysi-

cal record does not constrain the timing and geometry

of subduction and plate collision. Consequently, one

can only speculate on the relationship between the

alkaline magmatism and subduction along the Ecua-

dorian margin during Cretaceous times.

The occurrence and generation of the OBB alkalic

intra-plate magmatism in the Oriente Basin could be

explained either by the interaction of a deep mantle

plume or hot-spot [12–14], by rifting [16], slab win-

dow formation [17,18] or a droll-backT [11] mechan-

ism. Any model for the magmatic and tectonic

evolution of the Ecuadorian Cretaceous Oriente

Basin has to fit with the following observations: (i)

the volume of eruptive products generated by the

Cretaceous magmatic event is small; (ii) the geochem-

ical and isotopic OIB-type signature is consistent with

an asthenospheric source with no evidence of a com-

ponent from the subducting slab; (iii) paleo-Triassic

and Jurassic rift structures, inverted in a transpressive

regime during Cretaceous times, exerted a tectonic

control on the location and occurrence of major erup-

tive sites; and (iv) magmatism migrated to the SSW

along the active Central Corridor, starting in the

Albian (~ 110 Ma), in the northern part of the Ecua-

dorian Oriente Basin and finishing during the Campa-

nian (~ 80 Ma), in the west-central part.

The present-day back-arc position of OBB vol-

canism suggests that it erupted in an extensional

regime. However, as there is no evidence of there

having been enough lithospheric extension to cause

decompressional melting during Cretaceous times in

the Oriente Basin, it seems unlikely that the alkaline

magmatism is associated with rifting. In contrast, the

major alkaline eruptive sites are controlled by trans-

pressive inversion of a rift [20]. In such system,

strike-slip dominant inversion, only few antithetic

faults can be reactivated in extension. The mantle–


plume model can explain the SSW migration of mag-

matism over time, as suggested by radiometric data

combined with biostratigraphy. However, the rela-

tively small volume of volcanism generated by the

OBB magmatic event (50–100 Km3) is not consistent

with this model.

Is it possible then that the OBB alkaline event is an

indicator of slab-window formation in the Ecuadorian

margin during Cretaceous times? The slab-window

A TRIASSIC-MIDDLE JURASSICMisahualli

Arc

Allochthonous Plate

beginning ofSlab Roll-Back

Slab Roll-Back

Possible Slabbreak-off ??

LATE JURASSICEARLY CRETACEOUS (130-120 Ma)

CRETACEOUS (110-80 Ma) Alkaline volcanism

Collision, end of subductionand active arc magmatism

B

C

subduction is abandoned

en

Fig. 9. Schematic model of the geodynamic origin and evolution of the intr

Jurassic rift system and post rift back-arc basin associated with the Jurassi

Chaucha–Arenillas) ([29,31–34]) and cessation of the active subduction sy

80 Ma), progressive slab roll-back and migration of unmodified asthenos

beneath the Central Corridor.

model would explain the generation of small volumes

of alkalic melts in an area not affected by major

lithospheric extension after the cessation of active

subduction processes. However, the main element of

the slab window model is missing in the Cretaceous

Oriente Basin. Slab windows are formed consequently

to ridge–trench collision [17] and there is no geologi-

cal evidence of this interaction during the Latest Jur-

assic–Earliest Cretaceous. In fact, during the

(Syn-Rift ) Triassic-Lower Jurassicand Jurassic Back-Arc Basin

Oriente Cratonic Basin

Thin-Spot(previous lithospheric thinning)

AsthenosphericMigration

d of Jurassic Back-Arc Basin

TectonicInversion

a-continental thin-spot along the Oriente Basin. (A) Triassic–Middle

c Misahualli arc [32]. (B) Collision of an allochthonous terrane (i.e.,

stem ([29,30,60]). Beginning of slab roll-back. (C) Cretaceous (110–

pheric material towards a region of previously thinned lithosphere


collisional event (~ 140–120 Ma) [29,31–34], subduc-

tion was merely blocked and abandoned along the

Ecuadorian margin. Therefore, the Cretaceous OBB

alkalic event is not likely to be related to slab-window

formation.

We propose that slab roll-back, associated with

asthenospheric migration towards areas of continental

weakness (previously thinned lithosphere), can

account for the characteristics described for the origin

of the Cretaceous intra-continental alkalic magmatism

in the Oriente Basin. Similar examples include the

cratonic Patagonia Plateau lavas [6] and basalts from

the James Ross Island–Antarctic Peninsula [9]. Fig. 9

illustrates this possible mechanism. After subduction

ceased (~ 140–120 Ma), either the relict slab material,

corresponding to the eastwards-directed leading plate,

was simply rolled-back or detached and sunk as a

result of the large density contrast between the sub-

ducted part of the lithosphere and the surrounding

asthenospheric mantle. In both cases, this resulted in

lateral and vertical migration of unmodified astheno-

sphere, which moved into the region previously

occupied by subduction-modified mantle wedge. Con-

sequently, considerable slab roll-back has occurred

since Early Cretaceous times and may have started

in the north allowing unmodified asthenospheric

material to upwell into the region underlying the

pre-existing Triassic–Jurassic paleo-rift or litho-

spheric pre-existing thin-spot [15]. This resulted in

partial melting and the subsequent input of magmatic

material into the northern part of the Oriente Basin.

Major tectonic inversion occurred during Upper Cre-

taceous times [20,27,60], reactivating pre-existing

extensional features within a dextral transpressive

strike-slip system. As a consequence of the lateral pro-

pagation of the transpressive inversion of the Triassic–

Jurassic rift, magmatism migrated to the SSW along

the Central Wrench–fault Corridor during the Late

Cretaceous.

Pre-existing structures clearly excerted a funda-

mental control on the generation of alkalic magmas

during the evolution of the Cretaceous Oriente Basin,

and focused partial melting by facilitating astheno-

spheric upwelling. Eventually, the Cretaceous alka-

line magmatic event was halted by the progressive

effects of east-dipping subduction that was renewed

further west in the Late Cretaceous after the accre-

tion of allochthonous terranes currently exposed in

the Cordillera Occidental (i.e., Pallatanga [38,39])

and the fore-arc [31,37], as well as the development

of the subsequent compressional Oriente foreland

basin system.

9. Conclusions

The Oriente Basin of Ecuador offers new evidence

of a Cretaceous intra-plate alkaline magmatic activity

associated with the evolution of the northwestern

margin of South America. The geochemical and iso-

topic compositions of these basaltic rocks are consis-

tent with an asthenospheric source unmodified by

subduction, similar to an OIB-type mantle source. A

slab roll-back mechanism, associated with lateral and

vertical asthenospheric migration, is thought to have

facilitated the generation of alkalic magma when sub-

duction was abandoned possibly after the accretion of

oceanic and continental allochthonous terranes

between the Late Jurassic and Early Cretaceous.

Pre-Cretaceous rift and back-arc extensional struc-

tures influenced the generation and location of magma

emplacement, by acting as pre-existing lithospheric

thin-spots, promoting asthenospheric upwelling and

partial melting. Cretaceous tectonic inversion in the

Oriente Basin resulted in the reactivation of exten-

sional features within a dextral strike-slip transpres-

sive system causing a south–southwestward migration

of magmatism within the Upper Cretaceous section

following the trend of the Central Corridor. East-dip-

ping subduction resumed in the Late Cretaceous and

placed the Oriente Basin into a compressional regime,

which resulted in a cessation of OBB magmatism.

Acknowledgments

We thank D. Geist, T. Toulkeridis, R. Fleming, D.

Quirk, M. Weber, R. Spikings and anonymous for

stimulating comments and informal reviews. We are

grateful to John Huard (Oregon State University) and

Bill White (Cornell University) for assistance in40Ar/39Ar radiometric dating and Sr and Nd isotopic

analyses, respectively. We thank the University Paul

Sabatier of Toulouse (France) and Petroproduccion,

especially Jorge Toro for valuable help and discus-

sion. This research was supported by Kerr McGee Oil


and Gas Corporation and by the Institut Francais de

Recherche pour le Developpement. We want to ded-

icate this study to our friend and colleague Michel

Monzier.

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